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﹏ヽ暗。殇╰゛Y 2023-06-04 13:54 9阅读 0赞

At the top of the electron microscope is the electron gun.

As we've discussed it has a filament and

then what's often called the Wehnelt cylinder.

And then an accelerator stack to accelerate the electrons as they come out

and head down the column.

And this is called the electron gun.

Next we have the first lens system, which is called the condenser lens system.

Complete with a set of deflectors and lenses, usually there's two called C1 and

C2, stigmators and an aperture.

The purpose of the condenser lens is to take the electrons coming out of the gun

and focus and direct them onto the sample.

So here I've drawn the sample coming in on what this looks like a spatula.

Here's the sample.

And a sample resides within the next lens system which is the objective lens system.

And the objective lens system has a pair of deflectors, the lens,

here the objective lens, stigmators, and an aperture.

The objective lens system produces a magnified image of the sample.

And that magnified image is further magnified

by the third lens system which is called the projector lens system.

It, again, has deflectors and then, usually, a number of lenses.

Here, I've drawn three.

The first two are often called intermediate lenses, and

the last one would be called the projector lens.

And it has a stigmator and again, an aperture.

Finally, the magnified image is sent through a final pair of deflectors

onto some kind of detector.

May, it may be a viewing screen or a piece of film or a camera or whatnot.

And the idea of having so

many lenses is that the electron microscope is a compound microscope.

Meaning that, given an object, the first lens might mag, produce,

a first real image of that object at perhaps a hundred times magnification.

2:10

And then the next lens system takes the first real image and

further magnifies it into a second real image.

And if the intermediate lens has a magnification factor of say, 20,

the second real image is now magnified by a factor of 2,000.

Then another lens system, the projector lens system,

can take that second real image and produce a third or final real image.

And if the projector lens system also has a magnification factor of a hundred,

then you can get final images with magnifications in the hundreds of

thousands.

In fact, most,

electron microscopes can deliver images at one to five million times magnification.

Now the planes upon which these images exist here,

here, here, and

here where the original object is are called conjugate planes.

Because the same wave function exists here, here, and

also here, and it is an image of the object that exists here.

Except that in each case it's magnified to a different factor.

And we call those planes conjugate.

Now returning to the full schematic of the column we see that each lens system

has each of the four elements of deflectors,

a lens, a stigmator, and an aperture.

In the case of the condenser lens system,

the deflectors are called gun deflectors, because their job is to take

whatever electrons are coming out of the electron gun and shift them.

And change their angle so

that they will come straight down the optical axis of the two condenser lenses.

4:04

And the stigmator is called the condenser stigmator and

the aperture is called the condenser aperture.

In the objective lens system, the deflectors are called beam deflectors,

Because they're deflecting the beam that is being directed onto the sample.

4:21

And then you have the objective lens, the objective stigmator, and

objective aperture.

In the projector lens system, the deflectors are called image deflectors,

because they receive the first image that's formed by the objective lens

system, and they move that image so that it passes directly

down the center of the optical axis of the intermediate lenses.

4:47

The stigmator it, for

reasons that we won't cover, is called the defraction stigmator.

And the aperture here is called the selected area aperture.

Finally, the last set of deflectors are called projector deflectors,

and they deflect the image onto whichever detector that you are using at the time,

for instance, a viewing screen.

These are the names of the knobs, and the currents that you can control.

Here I've added the names of the parameters in the knobs

that you get to change on the electron microscope.

The first thing you can control is the current passing through the filament,

which also controls how hot it gets.

And that knob is usually called filament, or

that parameter in the software is often called filament.

5:37

Next you can control the voltage difference between

the wehnolt cylinder and the filament and this is sometimes called bias or

sometimes it's called emission.

And it controls, generally, what percent of electrons that are coming

out of this tip are passed through here toward the accelerator stack.

5:57

Next in the accelerator stack, you control the voltage drop across the stack

and this is often called the high tension.

Following the gun deflectors, the first condenser lens

takes the electron beam that emerges from the accelerator stack.

And has perhaps been redirected by the gun deflectors and

the first condenser lens focuses it into a first spot.

And so the, to

control the current in that C1 condenser lens the knob is called spot size.

And perhaps counter-intuitively, high spot sizes correspond to a small spot.

6:47

Which is going to be more coherent,

6:55

because some of the electrons that are the most divergent,

won't actually be focused into that small spot.

On the other hand, a low spot size corresponds to a larger spot that has

more electrons in it, and it's brighter, but it's going to be less coherent.

7:15

The next condenser lens is called C2.

7:18

And it takes that first spot as the electrons emerge from that first spot,

and its job is to focus these electrons onto a particular position on the sample.

7:34

Now I drew it first like this where all of the electrons would appear to hit exactly

the same spot on the specimen, but in fact, usually you don't want

to focus your beam, and so it hits in exactly the same spot on the specimen.

Instead, you want to spread it out a little bit and

illuminate a region of the sample, big enough to fill your image.

More typically, what's done is to set the current in the C2 lens.

So that there is what's called a crossover point above the sample.

8:06

So this focal point is called a crossover.

8:13

Position and that's placed above the sample.

8:19

And as you strengthen the C2 lens,

that crossover point rises in the microscope and

you see the region of your sample that's being illuminated gets larger and larger.

8:33

On the other hand if you decrease the strength of the C2 lens then the crossover

point will, drop in the microscope until ultimately it could be focused

right on your sample, and you see the minimum sized spot that you can create.

Because of this, the current in the C2 lens, the knob there is called intensity.

Because it dramatically affects the intensity

of the beam that you see on the sample.

When you focus the spot to a small spot, it's very bright.

When you spread it out into a large area, it looks very dim.

And so they've named that knob intensity.

Unsurprisingly to control the stigmation of the condenser stigmator,

9:20

that knob is called condenser stigmatism.

9:24

Of the condenser aperture, you can control the size of that aperture, and

you have to center it before you can take good pictures.

9:33

Next we come to the beam deflectors.

And in analogy to gunshift and tilt, here the knobs

that control the currents and the beam deflectors are called beam shift and tilt.

The specimen is held by a specimen holder and

we'll talk more about how that's inserted and how its position is controlled.

But for now, let's just note, that what you get to control is the position of

the spe, of the specimen, both its height in the microscope, up or down.

And that's called the z height.

And also the tilt.

The specimen holders allow you to rotate the sample back and forth.

And so you can control the tilt and also its position within the microscope, up and

down and left and right and in and out.

Next, the current in the objective lens is called focus,

so to change that, you have to turn the focus knob.

10:33

And it controls, of course, as scattering emerges out of the sample,

the objective lens focuses that to a point.

10:45

Coincidence with the objective aperture.

And depending on the current in the objective lens

that point can be drawn up or down as necessary.

And we'll talk more about why you would defocus the microscope or

keep it closer to focus.

It's very important for image formation.

Again, unsurprisingly, the stigmation applied by the objective

stigmator is controlled by the knob's objective stigmatism.

And in the objective aperture,

you control the size of the aperture and its centering.

11:24

Next, for the image deflectors,

the knobs that control those currents are called image shift and tilt.

11:32

The currents in the intermediate lenses are controlled

together by the knob called magnification.

Now it's possible to set your microscope to give you independent control

of each of these lens currents.

By the way,

these lenses are called the intermediate number one, intermediate number two.

There can be more.

And then usually the projector lens is the last one.

And so, you can set the microscope to give you free control of these currents.

12:02

But to simplify the situation, the microscope designers

will have picked a set of magnifications, major step sizes.

For instance, 5,000 times magnification, or

maybe 32 times, thousand times magnification.

64,000 times magnification, perhaps 202,000 times magnification.

They've picked a set of magnifications.

And chosen currents for each of the lenses to,

so that together, they will produce that total magnification.

So all you have to do is change the magnification knob to choose between

these major step sizes.

And the currents in all the lenses will change.

Having a number of preset magnifications also allows the microscope

manufacturers to choose a set of lens currents in the different lenses

that will minimize the net rotation of the image as it comes through the cone.

Remember, that electron microscope lenses as they form an image,

it rotates as it moves through the lens.

And so, it's advantageous to be able to move up and

down through magnification without having the image rotate wildly.

And so by having these preset magnifications,

13:26

all of the lens currents can be chosen so

that each time the image is not rotating much from one setting to another.

Now the first time you start doing this, you'll notice that as you change

the magnification there are several regimes of magnification.

The lowest regime is called low magnifications and

the next one is usually called M and

then the higher magnifications are called SA mag-, the SA range.

13:59

And what's happening here is that if you're in the SA range,

all three of these lenses are on and

their currents give you whichever magnification you are seeking.

But if you want to drop to very low magnifications, for instance, the m range,

usually, is around 2,000 times magnification.

It turns out that the best way to get that kind of magnification is to turn off

one of the lenses.

So perhaps in the n mode it'll actually turn off intermediate lens number one

and so only intermediate lens two and the projector will be on.

14:39

Likewise, if you want to go to really low magnifications like for

instance 500 times mag.

14:46

The best settings may be to turn off both intermediate one and

intermediate two and leave on only the projector lens.

14:55

Now, when you go through these ranges, you need to be aware that switching ranges

can dramatically affect the microscope, because turning on or off a lens can,

change significantly how much heat is being produced by that lens.

So for instance, if you're working in SA mode, and your microscope is well aligned,

and then you drop to LM, two of the lenses will turn off.

They stop producing heat.

The cooling water is still passing through the microscope.

15:29

And so the, the whole microscope may cool down a little bit, and this will change

the resistivities of all of the coils and change the magnetic field somewhat.

So you will find that your microscope is more stable if you can maintain,

stay in the same range.

Finally, for the diffraction stigmator, obviously you get to control

the stigmatism there and the size and the position of the selected area aperture.

For the projector deflectors, usually you're just given choices of diverting.

The image either to a TV camera say that might be positioned here at the side so

you can choose the position that will divert the image to the TV, or

you can leave the image going straight forward onto the viewing screen or

perhaps cameras that reside below the viewing screen.

Now as you start to work within electron microscopy group

you'll hear people talk about aligning the microscope.

We're always talking about alignment.

And what it means to align a microscope is that when they build

the microscope, and assemble these lenses and

all of the components on top of each other, they try as hard as they can

16:46

to mount them in exactly along one axis.

But when you're talking about distances of nanometers it's impossible to have

each element perfectly physically aligned, and

that of course is one of the reasons why you have deflectors.

So for each imaging condition that we'll use in the microscope,

17:09

we can determine what deflector settings are needed

to take the emission coming from the filament.

17:18

And shift it and tilt it so

that it'll be coming right down the optical axis of the first set of lenses.

And then again, we may need to shift it and

tilt it so that it comes right through the optical axis of the objective lens.

And finally, we may need to shift and tilt it again so

that it comes right down the optical axis of the projector lenses.

So what it means to align the microscope is to find those

deflector settings in all these deflectors that will produce

a well-formed image that go through nearly the optical axes of all the lens systems.

And once those deflector settings are found, they can be stored.

18:01

Then to align the microscope,

one would change the magnification to a different magnification, and retune it and

find those deflector settings that achieved this same result,

and those would be stored for that particular magnification, and

so on through all the magnifications that might be used.

Now as the microscope is used each day those alignment parameters can vary for

a number of reasons.

One is, for instance, that the shape of the tip can change over time, e,

exactly which parts of that sharp tip the electrons are coming out of can change

subtly, the direction and the number of electrons coming from each position.

And when that changes, then you may need different gun deflector settings.

In addition, there can be slow changes in temperature in the room, or

perhaps in the cooling water.

And if the temperature changes in any position in the microscope,

then resistivity as of wires change, electric fields,

magnetic fields change, and it could need different alignment settings.

One of the alignments is called pivot points, and

each set of deflectors has a pivot point alignment.

19:20

What the pivot point alignment is about,

19:23

I'm going to draw another set of deflectors over here.

And remember that the purpose of a deflector set is to be able to take

radiation coming, a beam of radiation coming in any particular direction, and

move it and then untilt it, so

that it would come down right through the optical axes of some lens below it.

Now, in addition,

there are times when we want to deliberately tilt the incoming radiation,

so we might want to deliberately set the tilt over here or over here.

Or, we may want to shift the illumination

to a different position and then have it come straight down.

And so deflectors allow you to shift and tilt the incoming beam.

But of course a shift and a tilt

20:20

require changes in the currents of, of both deflector sets.

20:27

And ideally we would be able to ask for

a specific shift of the beam for instance, from say, here to over here.

And have that shift without any tilt being involved or vice versa.

We might like to be able to tilt the beam from straight down to coming at an angle

without any shift along with it.

And so what the pivot point alignment does is sets various

parameters that convert your command to deliver a shift or

a tilt into changes in the currents in these coils.

And the pivot points allow it to deliver a shift,

without any tilt, and a tilt without any shift.

And so that's called, aligning the pivot points.

21:19

Now alignment is different than calibration.

\[NOISE\] There is a number of software packages today that allow microscopes to

take images automatically and then move on to another specimen,

and focus and take another picture and then move on and

take another picture, all without any user needing to be there.

These packages require that certain commands in the microscope be calibrated.

And what this means, for instance, is that the software asks for

a beam shift of say 100 nanometers to the left.

22:01

It needs to know if it asks the microscope for

100-nanometer shift, how much shift is actually applied.

It may be less, it may be a little bit more.

And so most of these software packages we, you'll need to go through calibration

procedures where you ask the microscope for a certain shift or

tilt, and then measure what the microscope actually delivers, and

you can calibrate the response that way.

Now in all this work of alignment and calibration, an important issue

that comes up is hysteresis.

22:42

What hysteresis is, if we were to plot the magnetic field produced by a lens

as a function of the current going through that lens,

if we were to start at low current and a low magnetic field, we would see

23:00

that as we increase the current, the magnetic field would increase with it.

23:05

However, if from that condition we were to decrease the current, we would find

that the magnetic field didn't follow exactly the same curve on its way up.

So fo, moving in this direction it would follow this curve,

23:24

but if we move the current back, it follows this curve.

23:29

And so it isn't the case, that for a specific current, you have one and

only one magnetic field that's produced.

You might get a magnetic field of this on your way up.

But if you're reducing the strength of the lens,

you might get a magnetic field of that which is quite different.

And this is called hysteresis.

And it has to do with the memory of a metal

of what was the previous magnetic field.

If the, the metal is highly magnetized and then the current is reduced,

some of the, the ordering that produces the magnetic field will persist.

Now, what this means is that if you're using a microscope, and

you're using it at let's say, 5,000 times magnification and

then you move to 18,000 times magnification and

set up your conditions just right for imaging, but then you move higher to say,

32,000 times magnification and set up the imaging conditions in that situation.

And then you move back down to 18,000,

you might find that your conditions aren't right anymore.

And that's because this, the state of the microscope depends not

only on the exact currents in all the coils at that exact moment, but

also what the currents were in the states preceding, arriving to that situation.

And because of this, there's a button on the microscope which is called normalize.

25:01

And what normalize does, when you hit that button,

it will take many of the currents in, in the microscope.

25:09

And it will run the current up to a high value, then back down to a low value and

then back up to whatever situation you were in when you hit the button normalize.

And this is supposed to standardize the result so

that any time you go to a situation and

you hit normalize, it should recover to what you expect.

转载于:https://www.cnblogs.com/klausage/p/11507045.html

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